The invention generally relates to electronically controlled motors and, in particular, to a draft inducer system providing improved speed/torque control for a motor driving a fan for inducing a draft in an exhaust.
In a conventional furnace, natural convection currents move air through the exhaust outlet of the furnace's combustion chamber to carry away exhaust gases. The moving air further induces a draft in the combustion chamber for mixing oxygen with the fuel being burned in the chamber. Heat energy remaining in the exhaust gases, however, is lost to the atmosphere which decreases the overall efficiency of the furnace. The use of heat exchangers improves furnace efficiency by extracting additional heat from the exhaust gases before they are vented to the atmosphere. Extracting heat from the exhaust gases, however, reduces the natural convection currents which would otherwise carry the gases away. One solution has been to use a draft inducing fan to force the exhaust gases into the atmosphere.
A prior draft inducer control system for controlling the exhaust of a furnace includes a pressure transducer located in the vent adjacent the combustion chamber, a speed sensor for the motor and a torque sensor for the motor. These components vary the induced draft as a function of the pressure sensed in the exhaust outlet of the combustion chamber. Such a control system is disclosed in commonly assigned U.S. Pat. No. 5,075,608, the entire disclosure of which is incorporated herein by reference.
Application Ser. No. 08/025,371, the entire disclosure of which is incorporated herein by reference, provides improvements in draft inducer systems which beneficially increase furnace efficiency. Such improvements allow preselection of the motor's speed/torque and preselection of the magnitude of the induced draft to match the draft to the particular requirements and operating conditions of the furnace. Other improvements allow this preselection to be made quickly and economically during manufacture or installation of the furnace and provide greater versatility of the draft inducer control system in responding to various control signal conditions.
Further improvements provided by application Ser. No. 08/299,528, the entire disclosure of which is incorporated herein by reference, minimize the risk of overheating the electronic control of a draft inducer system by maintaining motor operation within a safe area. For example, a motor driving a draft inducer is generally able to provide relatively high torque at variable speeds. However, operation of the motor at relatively low speeds results in increased operating temperatures when torque is excessive or when the motor is otherwise operating under abnormal conditions. If the motor exceeds allowable operating temperatures, its electronic control can fail. Present motor systems provide various controls affecting motor speed, current and torque but often fail to provide adequate protection against the generation of excessive heat. The system of application Ser. No. 08/299,528 maintains motor operation within a safe area with respect to motor speed/torque and provides a maximum current limit as a function of motor speed thereby to maintain safe operating temperatures.
While such systems represent improvements, further improvements in draft inducer control systems, draft inducer apparatus, and methods of control and operation are needed to beneficially provide motor control for maintaining pressure across the heat exchanger at a desired level without the use of a pressure transducer for controlling motor speed. Improvements in sensing ignition in the combustion chamber are also needed since combustion decreases the density of the combustion chamber gases moved by the draft inducer fan which can affect motor speed/torque.